12.1. INTRODUCTION

The 21-cm hyperfine transition of neutral hydrogen was first detected in
the Milky Way in 1951 by Ewen and Purcell. Two years later
(Kerr and Hindman
1953),
the HI emission of the Magellanic Clouds was observed from
Australia. Largely as a result of the pioneering efforts of
M.S. Roberts, by the time of his comprehensive review
(Roberts 1975;
the reader is referred to this source for the early development
of the field), HI in about 140 extragalactic objects had been
detected. Since then, mostly with the Green Bank 91-m, the Nançay,
the Effelsberg, and more recently
the Arecibo telescopes, the line has been observed in emission in
thousands of galaxies, to distances 104 times greater than
that of the Magellanic Clouds, and in
absorption out to redshifts greater than 2. For the first twenty years,
single-dish telescopes provided the vast majority of the HI data. By the
mid-seventies, the techniques of spectral line aperture synthesis,
developed especially at Cambridge, Green Bank, and Owens Valley, bloomed
in full maturity at Westerbork and more
recently at the VLA. The contributions of single-dish instruments have,
however, remained important, especially for survey-oriented projects
that require high sensitivity observations of many noncontiguous regions
of sky, and for pure detection experiments.

Neutral-hydrogen concentrations, or clouds, provide the starting point
for the collapse of matter into stars. Abundant HI in a galaxy indicates
potentially active
star formation processes, while lack of HI is a guarantee of a barren
galaxy and an ineluctably aging stellar population. Thus, from an
evolutionary viewpoint, the role of HI is of primary
importance. Dynamically, that is, in terms of the fractional
contribution to the mass, the perspective is different and, as discussed
in detail in Section 12.5, quite
dependent on morphological type. The
Milky Way provides a reference point. Our Galaxy is an intermediate- to
late-type spiral, perhaps an Sbc.
The optical surface brightness of the disk decreases exponentially with
distance from the center, with one-half of the spatially integrated
luminosity falling within
an effective radius of 4 or 5 kpc; the luminosity of the entire disk is
about 1.1 × 1010L, twice
that of the nuclear bulge. Although ionized gas is found in the
halo, the bulk of the interstellar gas is in the disk; similarly to the
light, but with the exception of the very central regions, the total
hydrogen gas (HI + H II + H2)
component mimics an exponential disk, with an effective radius close to
that of the optical emission. While molecular hydrogen is the dominant
constituent of the
interstellar medium within the solar circle, atomic hydrogen
predominates towards the periphery of the disk. In fact, HI extends
farther out than any other galactic disk tracer. By mass, the atomic
hydrogen resides mostly in diffuse clouds and in the envelopes
surrounding molecular clouds. The thickness of the HI disk is nearly
constant within the inner 10 to 12 kpc, the averaged density falling to
half its
peak value at the plane at about 150 to 200 pc from it. A density cross
section perpendicular to the plane does, however, show weak, broad
wings, associated with
a warmer, less clumped HI component of the interstellar medium, more
than twice as thick as the main disk component. The total amount of
interstellar hydrogen in
the inner 16 kpc of the Galaxy is about 6 × 109M, with
roughly half in molecular
and half in atomic form. While the atomic gas extends well outside that
radius, the contribution of the gas (as well as that of all luminous
matter) to the total mass
decreases sharply with distance, and probably the total gaseous mass of
the Galaxy does not exceed 1010M. As
the luminous mass fades rapidly in the outer regions
of the Galaxy, the dynamical mass traced by the system of globular
clusters and by the dwarf spheroidal satellites of the Milky Way
continues to grow almost
linearly with distance from the center. Within 16 kpc from the center,
this mass is 1.5 × 1011M,
rising to about 1012M
inside 100 kpc. Atomic hydrogen thus
represents less than 10% of the total mass within the easily
discernible stellar disk,
and about 1% of the total mass out to 100 kpc. Its importance relative
to other constituents grows for galaxies of later morphological type
than our own; within
the region mapped by the HI in some extremely optically-faint blue
galaxies, HI can be the principal constituent of the visible mass. By
contrast, very early spiral
and lenticular galaxies frequently have less than 0.01% of their mass
in the form of neutral hydrogen gas, ellipticals sometimes even less.